Question

A body of mass 1kg is thrown upwards with a velocity of 10m/s comes to rest after moving up 4m. The work done by air drag in this process is

Answer 1

using work-energy theorem -

initial K.E  +  Work done by air drag =  final P.E

⇒ 1/2mv² + W  =  mgh

⇒ W  =  1×10×4 - 1/2×1×10²

         =  40 - 50

         =   - 10J

Answer 2

A body of mass 1kg is thrown upward with a velocity of 10m/s comes to rest after moving up 4m.

first, we have to find how much body will move without air drag.
acceleration due to gravity acts on body due to this body comes to rest after moving up h m from the ground.
e.g., v² = u² + 2as
0 = (10)² - 2 × 10 × h
h = 5m

here you can see body moves 1m less when air drag is appeared on it.
e.g., change in height due to appear of air drag , ∆h = 5m - 4m = 1m
so, workdone by air drag = mg∆h
= 1kg × 10m/s² × 1m
= 10 Joule

hence , workdone by air drag = 10J